In cases of inherited cardiomyopathy, the heart muscle thickens or dilates in the absence of obvious disease conditions (such as hypertension). Recent research into the causes of inherited hypertrophic and dilated cardiomyopathy has shown that inherited single-gene mutations of cardiac sarcomere proteins (proteins that are responsible for heart muscle contraction) may cause disease. The most commonly affected sarcomere protein is myosin heavy chain. Myosin heavy chain is the heart's molecular motor and mutations of myosin have been identified to cause hypertrophic (HCM) and dilated (DCM) cardiomyopathy. Each type of familial cardiomyopathy is characterized by distinct functional changes. The study of single-gene mutations that trigger heart disease by distinct functional impairments provides a unique opportunity for defining important molecules involved in disease progression. Results obtained here will examine how clinically relevant myosin mutations cause HCM or DCM and provide insight into mechanisms of heart disease. Although these monogenic inherited disorders account for a small fraction of total heart failure cases, study of cellular responses to such gene mutations are also likely to be relevant to more common forms of heart failure. [unreadable] [unreadable]